1. Field of the Invention
This invention relates to an index guided semiconductor laser device that attains laser oscillation at an extremely low threshold current level.
2. Description of the Prior Art
Conventional semiconductor laser devices are classified into two groups, gain-guided semiconductor laser devices and index guided semiconductor laser devices, according to their optical waveguiding mechanism. Index guided semiconductor laser devices are superior to gain-guided semiconductor laser devices in view of transverse mode stabilization that is important in practical use. Index guided semiconductor laser devices having a variety of structures have been proposed, typical examples of which are BH (buried heterostructure) lasers and VSIS (V-channeled substrate inner stripe) lasers.
FIG. 2 shows a conventional BH laser device, in which a double-heterostructure with a laser-oscillating active layer 24 sandwiched between the cladding layers 23 and 25 is formed into a mesa on a substrate 21 and a burying layer 27 having a low refractive index is buried outside the mesa. The BH laser device oscillates a laser beam according to an index waveguiding operation and has a low threshold current of 10 mA or less. However, if a proper refractive index is not applied to the burying layer 27 and if a proper width w is not applied to the waveguide, the device will oscillate in a high-order transverse mode. Thus, the BH laser device is disadvantageous in that it is restricted by production conditions. Moreover, in order for the BH laser device to oscillate in a fundamental transverse mode, the width of the waveguide must be set to be 2 .mu.m or less, which causes breakdown of the facets at a relatively low output power level, so that mass-production of the device cannot be attained and reliability of the device is decreased. The reference numeral 26 is a cap layer by which ohmic contact is achieved.
FIG. 3 shows a conventional VSIS laser device, which is produced as follows: On a substrate 31, a current blocking layer 32 is disposed. Then, a striped V-channel 30 with the width w is formed in such a way that it reaches the substrate 31 through the current blocking layer 32, resulting in a current path. Then, on the current blocking layer 32 including the V-channel 30, a cladding layer 33, a flat active layer 34, and a cladding layer 35 are successively formed, resulting in a double-heterostructure multi-layered crystal for laser oscillation operation. Even when the width w of the waveguide is set at a value of as large as 4-7 .mu.m, since a laser beam outside of the wave guide within the active layer 34 is absorbed by the substrate 31, high-order mode gain is suppressed and a high-order transverse mode does not occur. However, the threshold current of this VSIS laser device is 40-60 mA, which is extremely higher than that of the BH laser device. This is because current injected into the device is confined within the inner striped structure formed by the current blocking layer 32, but carrier injected into the active layer 34 diffuses into the outside of the active layer 34, resulting in carrier unusable for laser oscillation. The unusable carrier results in unnecessary light and/or generates unnecessary heat, causing an increase in the threshold current of the device and a decrease in reliability of the device.
To overcome the problems of both the BH laser device and the VSIS laser device, as shown in FIG. 4A, a structure of semiconductor laser devices in which grooves are formed on both sides of the V-channel of the VSIS laser device from the protective layer 6 to the current blocking layer 2 by an etching technique and subsequently filled with a multi-layered crystal having a reverse bias junction has been proposed. This semiconductor laser device is produced as follows: On a p-GaAs substrate 1, an n-current blocking layer 2 is formed, and then a striped V-channel 10 is formed in such a way that it reaches the p-GaAs substrate 1 through the current blocking layer 2. On the current blocking layer 2 including the V-channel, a p-cladding layer 3, an active layer 4, an n-cladding layer 5, and an n-GaAs protective layer 6 is successively formed. Then, grooves are formed on both sides of the area including V-channel 10, resulting in a striped mesa 11. The grooves are filled with a p.sup.- -type first burying layer 7 and a p-type second burying layer 8. A cap layer 9 by which ohmic contact is attained is formed on both the n-GaAs protective layer 6 and the second burying layer 8. This semiconductor laser device is advantageous in that carrier injected into the active layer 4 only diffuses within the striped mesa 11 and moreover a laser beam produced in the active layer is absorbed by the area outside of the striped channel of the n-current blocking layer, resulting in a suppression of the occurrence of a high-order mode. However, there is a difference in the crystal growth rate between the burying layers, so that the thicknesses of the burying layers of the reverse bias injunction portion, i.e., the thicknesses of the p.sup.- -type burying layer 7 and the p-type burying layer 8, in the area 12 at a distance from the striped mesa 11 become equal to the range of the carrier diffusion or less. Thus, there is a possibility that, as shown in FIG. 4A, leakage current I.sub.L flows from the mesa to the burying area outside of the mesa, resulting in a limitation of the decrease in the threshold current. When the crystal growth period is enlarged so as to obtain sufficiently thick layer thicknesses, as shown in FIG. 4B, a multi-layered crystal having a pn-reverse bias junction is grown on the mesa 11, whereby a flow of current that contributes to laser oscillation is prevented, causing a decrease in the device characteristics.